The goal of this research program is to forge an understanding of the role of neurotrophic phenomena in the initial formation, subsequent maturation and ultimate aging of the craniofacial apparatus. Specifically, this investigation will focus on the morphological interactions between the developing trigeminal motoneurons and the morphogenesis of the jaw muscles in the leopard frog, Rana pipiens. This myoneural system is of special interest due to the changing relationships between the trigeminal motor axons and jaw muscles necessitated by the degeneration of larval myofibers and formation of the adult muscle during metamorphosis. Three main lines of investigation are proposed: 1. First, I will study the earliest outgrowth of trigeminal motor axons into the first branchial arch by using retrograde tracers, and scanning and transmission electron microscopy to depict their relationships with the early muscle masses. 2. Next, I will analyze the cellular events associated with the metamorphic turnover of the jaw muscles by using E.M. autoradiography and scanning E.M. to portray the cell lines that give rise to the adult myotubes. 3. Finally, I will examine the redeployment of the trigeminal motor axons in order to ascertain the mechanism by which these axons switch from one peripheral target to another during metamorphosis. Results from the proposed studies will provide additional insight into the normal maturation of the jaw apparatus and will aid in understanding how the nervous system participates in the initial formation, subsequent maturation and aging of the jaw apparatus.